Distribution of local open-circuit voltage on amorphous and nanocrystalline mixed-phase Si:H and SiGe:H solar cells

Local open-circuit voltage (V-oc) distributions on amorphous and nanocrystalline mixed-phase silicon solar cells were measured using a scanning Kelvin probe microscope (SKPM) on the p layer of an n-i-p stricture without the top ITO contact. During the measurement, the sample was illuminated with a laser beam that was used for the atomic force microscopy (AFM). Therefore, the surface potential measured by SKPM is the sum of the local V-oc and the difference in workfunction between the p layer and the AFM tip. Comparing the SKPM and AFM images, we find that nanocrystallites aggregate in the amorphous matrix with an aggregation size of similar to 0.5 pm in diameter, where many nanometer-size grains are clustered. The V-oc distribution shows valleys in the nanocrystalline aggregation area. The transition from low to high V-oc regions is a gradual change within a distance of about 1 mu m. The minimum V-oc value in the nanocrystalline clusters in the mixed-phase region is larger than the V-oc of a nc-Si:H single-phase solar cell. These results could be due to lateral photo-charge redistribution between the two phases. We have also carried out local Voc measurements on mixed-phase SiGe:H alloy solar cells. The magnitudes of V-oc in the amorphous and nanocrystalline regions are consistent with the J-V measurements.